COMODO is offering a tutorial that explains how to export the digital certificate from the browser (Chrome, Firefox, Internet Explorer) into a .PFX file that may later be imported into the Trust Center of Outlook.
https://support.comodo.com/index.php?/Knowledgebase/Article/View/1004/66/export-certificates-windows
We strongly recommend using Microsoft Internet Explorer 11 (not: Edge) for requesting/collecting S/MIME certificates.